This invention relates to refrigeration systems and more particularly to a closed circuit refrigeration system utilizing coalescent/depth filters.
The cryogenic mixed gas refrigeration system is a familiar system and has been described in numerous prior art documents such as U.S. Pat. Nos. 2,041,725, 4,535,597, 4,597,267, 4,689,964 and 5,161,382 and ASHRAE Refrigeration handbook, 1998, section 39.2. The limited application to which this technology has been put to is in part due to a number of sufficient shortcomings in the present systems. Prior art xe2x80x9cauto-cascadexe2x80x9d refrigeration systems have shown mixed gas systems to be an effective method of extending the normal temperature range of a refrigeration system with a single compressive step. Such systems are capable of cryogenic temperatures as low as xe2x88x92160xc2x0 C. By using a mixture of gases with differing thermodynamic properties, the components of which under compression may be preferentially separated on the basis of phase, provide intermediate cooling of the discharge gas system.
A mixed gas refrigeration system can be described as an extended multi-zone economizer where high pressure gas discharged from the compressor is cooled by low pressure returning gas via a heat exchanger into which condensed liquid refrigerant is evaporated through a metering device such as a capillary line or a thermal expansion valve. The phrase change from liquid to gas serves to cool the discharge gas stream further.
Prior art cryogenic systems contain a mixture of gases, which are sequentially condensed and extracted into the return gas stream to cool the discharge gases. Currently disclosed systems use continuous tube-in-tube heat exchangers and tangential/vortex type phase separators placed at suitable points along the length of the heat exchanger.
The most prevalent application of the prior art technology has been its use in xe2x80x9ccryogenic water vapor pumpsxe2x80x9d. These are a type of vacuum pump commonly associated with the industrial applications of vacuum, e.g., coating of plastics/paper and the manufacture of semiconductors. Such systems are used to preferentially pump water vapor from high vacuum systems through the trapping of water onto a copper or stainless steel Meissner coil placed inside the vacuum chamber. The advantage of such systems has been their very fast water vapor pumping speeds. Advances in design have allowed the fast cycling of such systems through the common refrigeration practice of direct injection of hot gases into a Meissner coil. Common applications of cryogenic water vapor pumps are thin-film coatings and the processing of semiconductor devices.
During the cool-down of an xe2x80x9cauto-cascadexe2x80x9d system from ambient, almost all of the refrigerant charge exists in the gaseous phase, and consequently the gas flow rates are high. As the system cools to its equilibrium cryogenic temperature, certain components condense and are separated and returned through capillary lines to affect cooling of the oncoming gas stream. At equilibrium, flow rates are greatly reduced. The reduction in gas flow increases with distance from the compressor. Thus, to be efficient, any device designed to separate the two-phase components (gas/liquid) must be capable of operating effectively in two differing temperature/pressure/flow regimes. Prior art systems have employed either impingement or centrifugal (vortex) separation processes. Both separation methods operate at around 80% separation efficiency under optimal conditions. Often the two separation methods are combined which increases the operational range of the hybrid device but maximal efficiency is always compromised.
Two of the greatest technical challenges which face the engineer of these systems are the efficient separation of condensed components from the gas stream and prevention of contamination of the cryogenic parts of the system with compressor oil or less volatile components of the gas mixture. To be able to achieve the separation of condensed from non-condensed refrigerants has proved to be the limiting factor in the widespread commercial application of this technology.
The whole system must be capable of operating over a very wide range of temperatures, gas flows and pressures which exist in the system between start and achieving a cryogenic equilibrium. At start-up, the gas mixture can be deemed to be homogenous throughout the system and at high temperatures and pressure. Since all of the components are in the gas phase, the velocity of the gas is high. High gas velocities are ideal conditions for the impingement type of phase separator.
Once the system cools, the less volatile components are removed and returned to the compressor by being evaporated into the suction line further cooling the discharged gas and ultimately causing the condensation of further component. At equilibrium, each separation point corresponds to the corresponding temperature of a component gas, which is subsequently colder than the previous point. At this point in the cycle, the system is at low temperature and pressures and the gas velocity has dropped as a result of most of the gas charge being liquefied. A further consequence is that the composition of the discharge line changes with distance from the compressor.
The vortex separator (cyclonic) has been favored as it provides a lower pressure drop than mesh or sieve impingement types. The vortex type of separator separates droplets on the basis of centrifugal force. It therefore favors larger droplets moving within a high velocity gas stream. This is ideal just after start-up and at points closer to the compressor where gas velocities and mass flows are higher. However, their efficiency is greater compromised as the system cools and becomes cryogenic. The impingement filter has some similarities to a coalescent filter. However, the mean free path is small and the effective pore size large. Impingement phase separation works at low gas velocities where the droplets may have an increased residence time. Because of the opposing properties of a vortex and impingement phase separation, it is common to have both principals within the same separator.
Another principle difficulty encountered with a cryogenic auto-cascade system lies with the fact that to achieve the low temperatures gases with low boiling points such as methane or nobles gases must be used. Such gases are well above their critical temperature at normal temperatures. They thus follow the Boyles Law behaving as ideal gases where PV=NRT.
Since an auto-cascade refrigeration system as described is a closed system, the volume of system V, quantity of gas N and by definition the gas constant R do not change.
The change in state, which an ideal gas undergoes during compression, may be described by       P1    T1    =      P2    T2  
Here pressure P and temperature T are expressed in absolute units (pa and xc2x0 K)
Typical refrigeration compressor operating compression ratios are between 10:1 and 20:1. In such a system compressing an ideal gas (i.e. one above its critical temperature) would cause the temperature of gas discharged from the compressor to increase by several hundred degrees Kelvin. This is far in excess of the capabilities of commercial compressors.
A solution is to use an agent to quench the discharge gas temperature. The basis of the effect lies in the fact that its boiling point is sufficiently high that it only changes from liquid to gas at discharge temperatures and pressures encountered within a typical refrigeration system. The change in state from liquid to gas absorbs a large amount of energy suppressing any adiabatic temperature increase caused by the compression of and ideal gases to achieve low compressor discharge temperatures.
Prior art has shown the use of refrigerant R123 to be effective in controlling gas discharge temperatures in large conventional refrigeration systems. The use of R123 as a chloro-carbon has been shown to cause damage to tropospheric ozone layer.
Another difficulty with auto-cascade systems is the fact that they rely upon a large compressor displacement and a complex interrelationship between each of the heat exchangers and the liquid phase metering devices, which are almost invariably, copper capillary lines. The gases used which follow Boyles law are subject to very high degrees of adiabatic heating when compressed. This becomes a critical problem in autocascade systems because of the very high compressor displacements compared to the overall net cooling effect.
Thus at start up discharge pressures and temperatures rise rapidly. The issue of reducing temperature rise in discharge gas from the compressor by the addition of certain components is addressed by prior art. Pressure of gas upon start up would exceed the working limits of commercial compressors. This is particularly true for hermetically sealed compressors, which are enclosed within a pressure vessel having strict pressure capabilities. In small and medium sized systems hermetic compressors are favored because of their durability and leak tightness. These are both highly desirable and critical requirements for cryogenic systems.
The problem of hermetic compressors in the context of auto-cascade systems is that the large gas volume of their integral pressure vessels is immediately available on start up causing disastrous pressure increases within the system. Various solutions to this problem have been employed by prior art. Most commonly, the use of a reservoir to temporarily increase the working volume of the system has been employed in a number of commercially available systems. The disadvantages of these systems have been cost and difficulties in satisfying legislative requirements with regard to pressure vessels. Motor speed control of 3 phase compressors has also been evaluated; however they have proved to be less effective in autocascade refrigeration systems since the system requires a certain gas velocity to facilitate the separation of the liquid from the gas phase of the system.
All prior art systems have used low internal volume semi-hermetic or open compressor types coupled with an expansion tank.
A typical prior art auto-cascade system is shown in FIG. 1 where to achieve a sufficient gas charge for cryogenic operation around half of the gas is pumped into a large storage tank 3 through a line directly connected close to the compressor discharge. The tank is typically over 3 times the total internal volume of the rest of the system. When the discharge pressure has dropped, valve 1 is opened and gas is reintroduced into the system close to the compressor inlet. The removal of the bulk of the charge means there is little gas available for suction cooling of the compressor. Secondly the gas contained in the buffer tank is not subject to any cooling and therefore adds a considerable load into the compressor on the opening of valve 1.
The present invention relates to a new application within a closed circuit refrigeration system of coalescent/depth filters to provide the almost total removal of liquid refrigerants from a refrigeration gas stream. The liquid refrigerant so extracted may then be returned to the suction side through a metering device such as a capillary line to provide intermediate cooling of the oncoming discharge gas. The efficiency of the coalescent membrane filter enables the use within an auto cascade system of short path plate heat exchangers. As contrasted with prior art systems with phase separation methods which have a continuous (i.e., open) path between each of the temperature regimes within the discharge line of the system, the present invention exploits the properties of coalescent/depth filters to place a physical barrier between the differing temperature/pressure regimes on the discharge side of the system. The point of placement of these coalescent filters at a point past the condensation point of a particular component of the mixture is critical.
The invention is a key improvement in the design of cryogenic auto cascade refrigeration systems since the plate heat exchangers used are significantly more efficient than the shell and tube solutions used in the prior art. The use of plate heat exchangers in such systems has been precluded by their short path length, which has resulted in oil migration into the cryogenic parts of the array, which in turn has led to unacceptable reliability.
The present invention relates to both the combination of the plate heat exchangers and coalescent phase separators assembled either together or separately within a cryogenic mixed gas refrigeration system. The invention provides a way in which these elements may be combined to form a functional element and how such elements may be joined in such a way as to form a highly efficient cryogenic refrigeration system.
The invention also covers specific elements of the control and design of such a system that takes advantage of the core technology as described to be an unique and effective method of generating cryogenic temperatures in a single compressive step.
Another feature of the invention is the provision of a start valve in a line connecting the gas discharge path and the return path close to the cryo-coil which enables the use of hermetic compressors and eliminates the requirement for supplementary pressure vessels within auto-cascade cryogenic refrigeration systems. This start valve controlled through the combined or separate measurement of gas temperature and pressure permits reduction of the pressure and temperature of gas during the start up and initial cool down phase within auto-cascade systems. The invention increases gas velocity during start up increasing heat transfer rates. The effectiveness of the system allows the use of the fully hermetic compressors without the use of large buffer or reserve pressure vessels in which the bulk of the systems gas charge is stored during start up and the initial cool down from ambient.
As an additional feature of the invention, a class of volatile liquids, normally used as industrial cleaning agents, is used in the refrigeration system.